1. Field of the Invention
The present invention relates to a process for producing cyclic alcohols by a catalytic hydration reaction of cyclic olefins in the presence of an oil phase containing a cyclic olefin, an aqueous phase and a catalyst, wherein the unreacted cyclic olefin is recycled.
2. Description of the Related Art
Production of cyclic alcohols by using a crystalline metallosilicate as a solid catalyst in the hydration reaction of cyclic olefins is discussed in many literatures and patents. For instance, JP-B-2-31056 proposes a method for producing cyclic alcohols by hydrating cyclic olefins using as catalyst a crystalline aluminosilicate having a primary particle size of not greater than 0.5 .mu.m. This publication teaches that in case a crystalline aluminosilicate having a primary particle size of not greater than 0.5 .mu.m is used as catalyst, there are formed two phases, i.e. an oil phase and an aqueous phase, in the reaction solution, with the crystalline aluminosilicate present in the aqueous phase, when the cyclic olefin to water weight ratio is in the range of 0.001 to 100. It also states that the cyclic alcohol produced from this reaction mostly exists in the oil phase, and the cyclic olefin and cyclic alcohol mixture obtained from the oil phase can be easily separated to obtain a cyclic alcohol because of large difference in boiling point. There is shown as a typical example of production method a process which comprises drawing out part of the two-phase reaction solution continuously, allowing it to stand to cause layer separation, taking out the oil phase from the upper layer, and yielding a cyclic alcohol from this oil phase by distillation or other means, while recovering and recycling the unreacted cyclic olefin.
It is known that in the production of cyclic alcohols by the hydration of cyclic olefins, trace amounts of the isomeride of the cyclic olefin and the hydrate thereof are formed as byproducts (See, for instance, JP-A-60-104028). JP-A-4-41448 notes that the presence of trace amounts of these byproducts in the cyclic olefin and cyclic alcohol mixture withdrawn from the hydration reactor poses a serious problem when the cyclic alcohol is separated in a distillation column and the residue is recycled continuously as the starting material for the hydration reaction. Specifically, said cyclic olefin and its isomeride, and said cyclic alcohol and the hydrate of its isomeride are all close to 1 in relative volatility, so that when their mixture is separated in a distillation column, the hydrate of the cyclic olefin isomeride stays as an impurity in the cyclic alcohol while the isomeride of the cyclic olefin remains in the residue, which is recycled as the starting material for the hydration reaction. Therefore, as such operations are continued, the cyclic olefin isomeride is accumulated in the starting material for the hydration reaction, and also the hydrate of such cyclic olefin isomeride in the reaction-product increases, causing excessive reduction of purity of the produced cyclic alcohol. So, in JP-A-4-41448, in order to obtain a high-purity cyclic alcohol, it is proposed to control the concentration of the cyclic olefin isomeride in the residue recycled as the starting material for the hydration reaction. According to this method, the purity of the produced cyclic alcohol is claimed to be 99.73 wt % after 20 hours of the reaction in Example 1.
According to the knowledge of the present inventors, however, it was found that when the above reaction is carried out industrially for a long time, there takes place a reduction of purity of the produced cyclic alcohol even if the isomeride of the cyclic olefin and its hydrate are removed sufficiently, although this was not called to account in the above-mentioned JP-A-4-41448 because of the short operation time.
An object of the present invention is to provide a process for producing cyclic alcohols through distillation, which process enables continues long-time operation and is capable of producing a prescribed amount of high-purity cyclic alcohol, with its impurity concentration sharply reduced, i.e. in a high yield.
As a result of extensive studies for solving the above problems, the present inventors found for the first time that the cause of the reduction of purity of the reaction product cyclic alcohol resides in the impurities having a boiling point between the boiling point of the cyclic olefin and that of the cyclic alcohol, which has not been called in question in the short-time catalytic hydration reactions. Said impurities are accumulated continuously in the mixed oil phase composed of the unreacted cyclic olefin and cyclic alcohol. They are small in amount in the initial stage of the reaction, but they are concentrated with time to an unignorable level. Increase of the accumulation of said impurities causes a corresponding increase of the inclusion of the impurities in the purified cyclic alcohol. Said impurities in the mixed oil phase are recycled along with the unreacted cyclic olefin from a part above the liquid feed part of the distillation column to the reactor, but since the yield of the cyclic alcohol depends on the amount of the cyclic olefin supplied to the reactor, and the like, the increase of the concentration of said impurities, namely the decrease of the concentration of the unreacted cyclic olefin recycled to the reactor, gives rise to the problem that the yield of the cyclic alcohol is reduced unless the operating conditions are changed. As means for increasing the purity of the purified cyclic alcohol, a method is known in which the heat duty of the reboiler used in distillation separation is elevated to remove said impurities in the cyclic alcohol. This method, however, involves the problem to cause an increase of heat duty itself, and is also incapable of reducing the amount of the impurities in the recycled cyclic olefin.
Further researches by the present inventors have led to the disclosure of the fact that in the process for yielding a cyclic alcohol, when the distillation residue separated from the cyclic alcohol is supplied to another distillation column for removing the impurities having a boiling point between the boiling point of the cyclic olefin and that of the cyclic alcohol so that the unreacted cyclic olefin cleared of said impurities will be recycled, it becomes possible to run the operation continuously for a long time, the concentration of the impurities is remarkably reduced without increasing the heat duty applied to the distillation column, and a high-purity cyclic alcohol can be obtained without reducing the production and yield of the cyclic alcohol. The present invention has been completed on the basis of the above finding.